Wireless operators of communication networks often have radio cell site solutions that include many different frequency bands. This requires multiple radio base station units on the same cell site to support each frequency band. There is an increasing demand for a single radio base station that can operate in more than one frequency band in order to reduce equipment and operational costs. In addition, future standards, such as Long Term Evolution-Advanced (LTE-A), will enable users to simultaneously transmit and receive across more than one band to achieve a faster data rate. Therefore, having a single radio unit with multiband capability can be very advantageous. However, due to the narrowband nature of conventional transmitter architecture and radio-frequency (RF) components, it is recognized herein that conventional transmitters are not sufficient for optimal multi-band operations.
In a single-band base station, a typical transmitter subsystem block could be shown by FIG. 1. The conventional transmitter includes a driver 12 to amplify a low power RF signal to an adequate level and a narrow band power amplifier 18 to further amplify the RF signal. The conventional transmitter may also include a coupler 30 that samples the RF signal from the power amplifier 18 and sends it back to a feedback receiver for power control and digital pre-distortion (DPD) linearization. A circulator 40 protects the power amplifier from any reflected signals by providing isolation. A duplexer 50 additionally filters out any out of band emissions and enables the receiver (through low noise amplifier 70) and the transmitter to share the same antenna 60 by providing isolation between the two paths.
Many of the components, such as the power amplifier 18 and circulator 40, are inherently relatively narrowband, e.g., having a fractional bandwidth less than about 20%. Some studies have involved wide-band, highly efficient, power amplifiers. A multi-band power amplifier that operates in two or more bands is also possible, but not necessarily for broadband use, i.e., with fractional bandwidths greater than about 20%. However, it is recognized herein that constructing a circulator for broadband use can be difficult due to the inherent nature of the resonance of the magnetic material in the circulator or isolator.
There have been several proposals for multiband transmitter architectures. The first, shown as a dual band transmitter in FIG. 2, has two independent transmitter paths—one for each frequency band. The transmitter has drivers 12a, 12b, narrowband power amplifiers 18a, 18b, couplers 30a, 30b and circulators 40a, 40b. The signals are then combined at the band combiner 80 (duplexer) for transmission on antenna 60. An advantage of this architecture is that each transmit path is optimized specifically for one band; performance for multiband is as good as a single band. Any band combinations at any frequencies are possible. The disadvantages include the large size required for two transmit paths, including two power amplification paths. This is a challenge when designing for a small radio that can be mounted on the tower top. Especially for radio designs that support Multiple-In-Multiple-Out (MIMO) techniques, this will become expensive and bulky.
A second multiband transmitter architecture option, as shown in FIG. 3, uses one transmit path that is capable of operating in more than one band, using a wideband power amplifier 20 and a wideband circulator 40. The power amplifier 20 generally has a fractional bandwidth that is wide enough to operate in more than one band. Typically the power amplifier 20 can operate within neighboring bands occupying approximately 20% fractional bandwidth or more. The circulator 40 will also need to be wideband to provide good return loss and isolation protection to the power amplifier 20. Dual-band filter 90 filters the signal. This architecture allows for a smaller size radio since only one transmission path is required. However, a disadvantage of this broadband single transmit architecture is the RF bandwidth limitation of the power amplifier 20 and especially the circulator 40. The power amplifier can be constructed for broader bandwidth or designed for two or more bands. However, the circulator will be the main limitation, as it is generally limited to about 20% fractional bandwidth. More than 20% bandwidth is possible, but this requires sacrifices in performance, including poor return loss to the power amplifier. Basically, the isolation in such a circulator is poor and the large magnetic disks occupy a large area of printed circuit board (PCB) space. Further, multi-band operation for scenarios where the bands have a frequency separation of more than one frequency octave is not known to be possible at all.
It is recognized herein that existing circulators are insufficient for many current and future multiband purposes. A wideband circulator is limited by the magnetic material. Conventional circulators do not have a wide enough bandwidth to cover dual bands that are more than 20% fractional bandwidth apart. Complexities in design of broadband circulators may require expensive magnetic materials and novel design techniques. Furthermore, broader band circulators require performance sacrifices such as poor isolation, mismatch and insertion loss, which result in poor unpredictable power amplifier performance and higher power consumption.
It is further recognized that there may be poor performance due to aliasing within the transmit observatory receivers due to the harmonics of one or more bands that are further spaced apart in frequency.